The present invention relates to a scanning type projector which has a wide field and projects with high resolution power.
As a conventional projector which projects with high resolution power, a stepper is known in which a refractive optical system is used to successively project in small division to expose. The stepper is capable of reduction and enlargement, in which a numerical aperture (NA) can be provided to be large.
However, when it is intended in this optical system to ensure a large field, a projecting magnification becomes one-fold, and it is difficult to ensure a field not less than 150.times.150 mm due to the occurrence of the extra-axial abberation. For example, there is such a problem that it is not suitable for an exposing apparatus for an LCD (liquid crystal display device) of 300 mm.times.210 mm square to be used for lap-top type word processors.
As another example of the conventional projector which projects with high resolution power, such one of the scanning type is known which is disclosed in the official gazette of Japanese Patent Publication No. 51083/1982, and constituted as shown in FIG. 7 such that a concave mirror 100 and a convex mirror 102 are opposed and arranged.
In this optical system, an object to be projected and an image focusing position are arranged at symmetrical positions using the optical axis 104 as the center, and a corrected abberation region 106 in which an abberation is corrected is formed at a ring-shaped portion using the optical axis 104 as the center. The luminous flux to be used for projection is only a luminous flux which passes through the corrected abberation region 106 limited by a visual field diaphragm (not shown) in an illumination optical system arranged at a conjugated position with respect to the object to be projected. The scanning is performed by moving the object to be projected and a photosensitive material arranged at the image focusing position in each arrow in a synchronized manner.
In this case, when the maximum value of distortion of the optical member is Pv, the abberations generated by this Pv are as follows: EQU 2.times.Pv for the distortion of the reflection plane, EQU (n-1).times.Pv for the distortion of the refraction plane.
Here, n is a refraction index of the lens of the refractive optical system, and there is generally given n=1.5, so that the occurrence of the abberation at the refraction plane is (0.5.times.Pv) with a sensitivity which is 1/4 times the reflection plane, and hence it is understood that the refraction plane is more advantageous with respect to the large scale formation of the optical member.
On the other hand, when a scanning type projector having a wide field is used as an exposing apparatus for LCD (liquid crystal display device), masks to be used are expensive in their production cost, so that small one is desirable.
Further, when the resolution power of the optical system of the scanning type projector is considered, it is desirable that the numerical aperture NA is made large. In the conventional optical system shown in FIG. 7, the luminous flux which passes around the convex mirror 102 interferes with the luminous flux which is inputted into and reflected by the convex mirror 102, and the numerical aperture NA cannot be made large.